1. Field of the Invention
The present invention relates to apparatus useful for optical metrology. More particularly, the invention relates to a lens system which has a built-in reference surface and which provides an emerging wavefront of variable radius of curvature useful in an interferometer or differential autocollimator.
2. The Prior Art
The development of the laser has greatly expanded the utility of classical interferometers. The Fizeau interferometer, in particular, has become an extremely convenient and flexible instrument for a wide variety of optical metrology applications. A multiple beam spherical wavefront interferometer is discussed in detail in an article by Heintze et al in Applied Optics, Vol. 6, p. 1924, November 1967. A major difficulty arises with a spherical wavefront interferometer when slow, i.e., large focal ratio, elements or systems must be measured. In order to keep the distance between the article under test and the test apparatus short, the radius of curvature of the measurement wavefront must closely match that of the article under test. Therefore, the radius of curvature of the partially transmissive spherical reference surface must be selected to match closely that of the article under test. For precise optical measurements, a compact measurement set up is desirable both economically and technically because it minimizes the adverse effects of vibration and air turbulence.
Other types of apparatus have been used to test large radius optical elements and systems. For example, scatter plate interferometers and shearing type interferometers are two prominent means. However, these interferometers are not only difficult to use and align, but they are considerably less versatile than a Fizeau interferometer.
While these prior-art techniques are useful for some applications, they cannot be used in many optical metrology applications because of the specific, close match between the elements required to carry out the measurement and the parameters of the test article. To this end, a device is required for testing long radius of curvature optical elements and systems which does not require the specific, close match of expensive elements to the parameters of the test article.